The present invention relates to a method for extracting bitumen from an oil sand stream, in particular using a non-aqueous solvent. More in particular, the present invention provides a method for removing rejects from an oil sand stream.
Various methods have been proposed in the past for the recovery of bitumen (sometimes referred to as “tar” or “bituminous material”) from oil sands as found in various locations throughout the world and in particular in Canada (such as in the Athabasca district in Alberta) and in the United States (such as in the Utah oil sands). Typically, oil sand (also known as “bituminous sand”, “oil sand ore” or “tar sand”) comprises a mixture of bitumen (in this context also known as “crude bitumen”, a semi-solid form of crude oil; also known as “extremely heavy crude oil”), sand, clay minerals and water. Usually, oil sand contains about 5 to 25 wt. % bitumen (as meant according to the present invention), about 1 to 13 wt. % water, the remainder being sand and clay particles.
As an example, it has been proposed and practiced at commercial scale to recover the bitumen content from the oil sand by mixing the oil sand with water and separating the sand from the aqueous phase of the slurry formed. Disadvantages of such aqueous extraction processes are the need for extremely large quantities of process water (typically drawn from natural sources) and issues with both removing the bitumen from the aqueous phase (whilst emulsions are being formed) and removing water from the bitumen-depleted sand.
Other methods have proposed non-aqueous extraction processes to reduce the need for large quantities of process water. Examples of such a non-aqueous extraction process are disclosed in e.g. U.S. Pat. No. 3,475,318 and US 2009/0301937, the teaching of which is hereby incorporated by reference.
In non-aqueous solvent extraction processes oil sand ore is mixed with a non-aqueous solvent containing stream thereby obtaining a solvent-diluted oil sand slurry. As downstream processing equipment typically requires particles above a certain maximum size to be removed, these bigger particles (also called “rejects”) are screened from this slurry.
A problem of non-aqueous solvent extraction of bitumen from oil sand is that any rejects being removed from the slurry need to be discharged to the atmosphere. Hence, the (non-aqueous) solvent content in the rejects needs to be reduced to a sufficiently safe level before the rejects can be exposed to the atmosphere. This problem is in particular pertinent in case a volatile solvent is used for the extraction of bitumen.
A further problem of non-aqueous solvent extraction of bitumen from oil sand is the provision of an effective seal between the usually slightly pressurized (typically volatile hydrocarbon) solvent processing environment and the atmosphere, to prevent the venting to the atmosphere of the non-aqueous solvent (vapours) as used for extracting bitumen from oil sand. Transporting rejects through such a seal is a technically challenging operation.
Typical examples of equipment used for such solids transport operations are rotary star valves, lock hopper systems and the like. However, the nature of the rejects as obtained in an oil sands solvent extraction process, in which relatively large rocks can be present, cause significant issues in designing an economic, reliable and low-maintenance system, in particular whilst avoiding the venting of the non-aqueous solvent to the atmosphere.
It is an object of the present invention to solve or at least minimize these problems.
It is a further object of the present invention to provide a method that allows for the integration of the removal of solvent and bitumen from rejects and the transporting of these rejects through a seal between the hydrocarbon processing environment and atmosphere.